How We See Dark Matter

Dan Coe
by Dan Coe
Astronomer

Planets, stars, buildings, cars, you and I, we are all made of the same basic stuff - atoms, the building blocks of matter. The late Carl Sagan famously said “we are star stuff,” as the heavy elements in our bodies were all forged in supernovas, the explosions of dying stars. In a real scientific sense, we are one with everything we see in the night sky.

We have since learned that everything we see is awash in another kind of matter, a “dark” matter, made of particles yet to be discovered. Dark matter is all around us, but we cannot see it. Some estimate that a billion dark matter particles whiz through your body every second, but you cannot feel them. We now believe that the universe contains five times more dark matter than ordinary matter. While we all may be made of star stuff, we find that the universe is mostly made of something very different.

Why do we believe that dark matter exists? How can we study something that we cannot see or even feel? And how can we unravel the universe’s greatest mystery - what is this dark matter?

The idea of dark matter was born at Caltech in 1933. (Just three years later, JPL would be born there as the “rocket boys” began their first launch experiments.) In observations of a nearby cluster of galaxies named the Coma cluster, Fritz Zwicky calculated that the collective mass of the galaxies was not nearly enough to hold them together in their orbits. He postulated that some other form of matter was present but undetected to account for this “missing mass.” Later, in the 1970’s and ’80’s, Vera Rubin similarly found that the arms of spiral galaxies should fly off their cores as they are orbiting much too quickly.

galaxy cluster
In this Hubble image, the galaxy cluster Abell 2218 reveals its dark matter by lensing background galaxies into giant arcs. Image credit: NASA/JPL.

Today dark matter is a widely accepted theory, which explains many of our observations. My colleagues and I at JPL are among those working to reveal and map out dark matter structures. Dark matter is invisible. But astronomers can “see” it in a way and you can too, if you know what to look for! For instance, if you have a wineglass on a table and you look through the glass, the images behind it are distorted. So too when we look through a dense clump of dark matter, we see distorted and even multiple images of galaxies more distant. Matter bends space according to Einstein’s Theory of General Relativity, and light follows these bends to produce the distorted images. By studying these “lensed” images, we can reconstruct the shape of the lens, or in our case, the amount and distribution of dark matter in our gravitational lens.

Our observations of dark matter in outer space force particle physicists to revise their theories to explain what we see. Hopefully through their efforts, physicists will soon produce dark matter in the lab, catch and identify a small fraction of that which passes through us, and ultimately explain the relationship between dark matter and “star stuff.”

    20 Responses to “How We See Dark Matter”

  1. James Turner Says:
    February 3rd, 2009 at 5:54 pm

    I am a 74 year old layman who delights in surfing the NASA website every evening.

    I find the subjects on the Universe simply thrilling and mindboggling.

    It appears that the Universe is expanding outward to possibly a never ending conclusion.

    It makes me wonder if it is possible that our Universe as we know it in the other direction towards the atom is not the final as we know it.

    Could it be possible that the atom is in itself a galaxy and electrons effectively planets of a minature solar system. And if so , is it possible that there could be futher ” galaxies ” below the minor electron that man has defined.

    In laymans terms , what proof does man have that the electron is the smallest particle ?

    Just wondering.

    Jim

  2. johnny Says:
    February 4th, 2009 at 10:46 am

    we must all be interconnected physically and metaphysically, they both exist as one. how can that be that all is one and one is all, then we must have the answer within one self? just as the saying goes, together everyone accomplishes more in other words, or e = mc squared.

  3. Nicholas Byrd Says:
    February 4th, 2009 at 5:27 pm

    Hello Jim,
    It is wonderful, as always, to see a “layman” interested in the universe and physics in general. The fact of the matter is that there is no known “bottom of the pool.” Smaller than electrons are quarks, quantum particles. These particles do not behave in the way normal matter does as we view it, they exhibit all sorts of unusual behaviors. The study would be called Quantum Mechanics, and is well worth your time to study. Much like the observable Universe life remains complex as you get smaller. You are a consciousness spawned by the flow of electrons though neural tissue, brain matter, in your body, living in a house, in a community, in a country, on the Planet Earth, in the Local Solar System, in the Orion Arm of the Milky Way Galaxy, in our Local Group, located in the Virgo Supercluster. Turn that viewpoint downward and you will find there is just as much to discover in the other direction. This is the gift on consciousness, enjoy it.

  4. Bobble1 Says:
    February 4th, 2009 at 7:56 pm

    I thought that quarks are generally thought to be smaller than electrons? I don’t think electrons are any longer thought to be the smallest particle, despite being taught that in the 1960’s.

  5. Bobble1 Says:
    February 4th, 2009 at 8:00 pm

    Doesn’t dark matter kind of defy what we were taught that matter was? Guess I’ve been out of school way too long.

  6. Ezra Abrahamy Says:
    February 5th, 2009 at 2:28 pm

    Does dark matter have “weight”? Can dark matter really be distiguished from non-dark matter? Or, is it just beyond the spectrums we’re used to “see” things with?

  7. johnny Says:
    February 7th, 2009 at 11:19 pm

    dark matter is made of quarks is it not?, is it not sub atomic particles with which we are speaking of when speaking of dark matter? yet they are one and the same. matter must have mass and acceleration, right? one can not come to the conclusion otherwise with out the fundamental physicical explanation or equation of matter, which is it must have mass and acceleration to exist at all.

  8. Stephen Says:
    February 10th, 2009 at 11:45 am

    Dan Coe writes, “Later, in the 1970’s and ’80’s, Vera Rubin similarly found that the arms of spiral galaxies should fly off their cores as they are orbiting much too quickly.” How does Rubin’s finding discount the gravitational effects of massive black holes at the center of galaxies, thus implicating dark matter as the binding mass?

  9. Michael Gmirkin Says:
    February 12th, 2009 at 1:25 am

    Is dark matter really even necessary? It was certainly never predicted and has never been bottled nor identified in a lab.

    In the article you state:
    “The idea of dark matter was born at Caltech in 1933. (Just three years later, JPL would be born there as the “rocket boys” began their first launch experiments.) In observations of a nearby cluster of galaxies named the Coma cluster, Fritz Zwicky calculated that the collective mass of the galaxies was not nearly enough to hold them together in their orbits. He postulated that some other form of matter was present but undetected to account for this ‘missing mass.’ Later, in the 1970’s and ’80’s, Vera Rubin similarly found that the arms of spiral galaxies should fly off their cores as they are orbiting much too quickly.”

    Is it not possible that Zwicky et al missed an opportunity by not revisiting foundational assumptions, e.g. gravity as the sole arbiter of galactic motions?

    Popper would say that Zwicky and Rubin mutually falsified gravitational models of gross galaxy motions. Two options I could see at that juncture. A) Admit that gravitation-only models of galaxy rotation had been strictly falsified and move on to figure out what ACTUALLY controls galaxy rotation / structure or B) Invent and pepper in unpredicted ‘new physics’ (dark matter) to account for the glaring discrepancy between predictions from gravitational models and actual observation.

    It seems Zwicky et al opted for route B, and invented unobservED and apparently directly unobservALBE ‘dark matter’ as a galactic fudge factor to balance the equations. I generally find the invention of additional unpredicted entities an uncomfortable direction for science (especially considering Ockham’s [Razor] admonition: “entities should not be multiplied beyond necessity”).

    I wonder what might have happened had Zwicky et al opted for route A and declared gravitational control of gross galactic motions definitively falsified, a la Karl Popper?

    I suggest lightly that route A may be satisfied (without resort to ‘dark matter’ or unknown ‘new physics’, though based upon considerably different foundational assumptions) by Particle-in-Cell (PIC) simulations carried out by Anthony Peratt at Los Alamos in 1986 and submitted in a paper to IEEE Transactions on Plasma Science, entitled “Evolution of the Plasma Universe: II. The Formation of Systems of Galaxies.” The results appear to tentatively conform to galaxy rotation curves and account for certain galaxy morphologies depending on plasma / electrical starting conditions, as well as matching several perturbed / “peculiar” galaxies similar in form to a number observed in Halton Arp’s Atlas of Peculiar Galaxies, if I recall correctly (some of that may have been in part 1 of the 2 papers, both available by googling ‘Peratt download papers’; no quotes).

    Just another hobbyist, I suppose. Albeit one with a few alternative opinions.

    Warm regards,
    ~Michael Gmirkin

  10. Ricky W. Trimnal Says:
    August 4th, 2009 at 1:19 am

    How is it possible for so called dark matter to have an original date on actually what our telescope’s or other device’s are recording. How is it possible to detect to a science what we think we are seeing. Just because a System look’s distorted, does not neccessarily mean that we are looking at that object as we see it know. There may have been many different factor’s to change what we think we are seeing.

    Sinserely, Ricky W. Trimnal

  11. Rubin Sarmell Says:
    April 6th, 2010 at 3:46 pm

    Dark matter can simply be what we know as “virtual particles”.
    All this energy, that all of a sudden appears, and then disappears again.
    The entire system is a never ending bubbling event, that seeks some sort of balance within itself.
    Most seem to happen “out of space”, but cause things to happen here!

  12. mohsen(allen) karimian Says:
    April 10th, 2010 at 1:46 am

    In my opinion ,dark matter or dark energy is some kind of matters that made of the very microscopic particles that could not been observed by scientists yet , i am a person who live in iran i’m 40 years old i just have learned some news about researchs about universe but what american’s or other countries scientists have found about universe or dark matter is a tiny stone among an ocean.
    what is dark energy or dark matter? they are something that our visibility couldn’t determine.but they are exist. our knowledge about soul,spirit,fourth or more dimensions is as much as our knowledge about dark energy.

  13. Klaus Runge Says:
    April 13th, 2010 at 9:31 pm

    A quick conversation I had, and some thoughts.

    (9:02:14 PM) Klaus Runge: But I had a thought, and that was that dark matter may simply not respond to EM waves well. You see the assumption we make is that standard EM waves are the only possible waves that exist. They cover the wavelength of a bowling ball all the way up to high energy waves.
    (9:02:56 PM) Klaus Runge: They are how we view our world. But there simply may be another form wave and hence associated energy.

    (9:05:41 PM) Klaus Runge: Yes, Maxwells equations were thought up to fit an empirical set of data and observations (a simple EM wave). But all it would take is for another type of wave to exist, and hence another form of energy and associated mass. E=mc*2 may hold true and for EM wave E=h*freq. But another wave may have different energy and be invisible to us as we view our universe through EM waves.

    Someone may wish to look at whether another form of EM wave, or altogether different wave, may theoretically exist outside of Maxwells equations which as previously stated were created to fit empirical data gathered by observing standard EM waves.

    I hope this may stimulate some thoughts, and research.

  14. klaus runge Says:
    April 13th, 2010 at 10:04 pm

    A simple example (just an example) would be a wave that changes polarization or exist with a different polarization of the electric field. If E1M is a standard EM wave, then E2M might not be observable (where E2 is a different polarization angle other than E1M)
    One could even have E1ME2M with oscillating polarizations, which probably would also not be detectable.

    Polarization here is just an example of the many twists on conventional EM waves that may exist.

    There are many possibilities as this is not an area of my deep expertise, but I hope a theoretical physicist may consider such possibilities.

    Best Regards

  15. klaus runge Says:
    April 13th, 2010 at 11:27 pm

    A final thought is that dark matter may even behave like regular matter (as it bends conventional EM waves with gravity), but may emit energy in a different EM wave format.
    Thus not be directly observable, and not have it’s emissions detectable. Also energy energy stored in this alternate EM wave format would not be detectable, as technologies such as a photodetector or antenna may not generate an electron-hole pair/electron in response to an alternate EM wave

    So the problem may lie not so much in the dark matter itself, but rather in that our theory and understanding of EM waves is lacking. Therefore it is our observation of the universe that may be limited.

    Some food for thought.

  16. Ed Wagner Says:
    April 18th, 2010 at 12:00 pm

    I’m sure our understanding is lacking. I don’t see why it is not obvious that some kind of sound like waves should exist in dark matter. These may have much to do with the universe organization and change our understanding of the beginnings of the universe.

  17. johnny Says:
    May 12th, 2010 at 8:33 pm

    i have a theory on dark matter and energy. could it be possible that this is the remnants of the big bang and or could be the evidence of the time before our time, in terms of the beginning of our universe and the end of a previous universe, which may seem like eternity to us, so it is beyond what we really see or ever will see. so dark energy is the missing link to before we began the universe we are today.

    u=u1~i, where i is infinity u equals our universe, this could explain how no information is lost,, but recycled

  18. stubbler Says:
    August 15th, 2010 at 2:18 am

    i’m a 12th grader who is really intrested in space science

    why can’t it be the relatisvistic mass variation that holds the galaxies together rather than dark matter?

    does it really takes a matter to bend light why a strong magnetic or gravitational field do it?

  19. Naveed Shah Says:
    November 13th, 2010 at 9:21 pm

    Dear Dane Coe,
    I have read detail about it, but I have doubt in my mind that this is not the ultimate proof of dark matter existence. http://chandra.harvard.edu/photo/2006/1e0657/.
    My point is that galaxies moved away from each other, they left traits of matter in the open space, when these clusters collided (as shown in the picture) with each other the lit up space we see is that matter/gases, which lay scattered all over the place due to universe’s expansion. You need to clarify,
    1. Have you discovered this scattered gas, and saying it is dark matter.
    or
    2. You believe that this lit up is totally due to that unknown substance called dark matter and those scattered left over ions/gases/diluted matter is nothing to do with this.
    Please clarify.
    Theory of lenses also speaks of existing of space or medium not a matter.

  20. Dylan Says:
    November 11th, 2011 at 5:14 pm

    Dear Dane Coe,

    I am also a Layman with little education, but I have an extreme interest in dark matter. Please bare with me. Can dark matter be matter condensed in a black hole? I have read about massive black holes at the center of galaxies. Can these black holes be pulling in all dead matter from the galaxy and condensing it into Dark Matter? IS the force we see, Dark Energy bringing this dead matter to the center of the black hole? I have a theory that this is a cycle and after all mass is back in one spot it allows the Big Bang to happen.

    Thanks for your help with this. I look forward to your response

    Dylan Kenyon

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